The Implications of Fast Charge in Lithium Ion Battery Performance and Life: Cell vs. Pack

Abstract

Within the past decade, the average price for automotive lithium-ion battery (LIB) packs has fallen roughly by 80% [1]. This has played a substantial role in increasing the driving range of mass market electric drive vehicles (EDVs) and the demand for EDVs [2]. Additionally, the increasing availability of direct current fast charging (DCFC) stations is working synergistically in aiding EDV adoption and utility. For instance, a 25% annual increase in electric vehicle miles was documented in areas where 50 to 120-kW DCFC stations were available [3, 4]. Thus, continued DCFC network expansion, along with faster charging, could significantly increase the utility of battery electric vehicles and alleviate consumers’ range anxiety to a comfortable level. EDV charging speeds are not yet comparable to the fueling speed of conventional gasoline engines, which is typically less than 10 minutes [5]. Higher- power charging stations up to 400 kW are necessary to achieve a 10-minute recharge [5]. Additional challenges will be encountered in realizing this extreme charging speed, from battery cells to vehicle systems, and from charging infrastructure hardware to charging network economic feasibility. On the battery side, the increased charging rate associated with extreme fast charging could adversely affect battery performance and life (i.e., state of health [SOH]). Besides the cell-level aging, additional pack-level aging factors could come into play under fast charge conditions. Thus, it is paramount to understand the effects of fast charging on LIB’s SOH, from the pack to cell level and to identify the most critical factors affecting battery SOH. This understanding would benefit battery developers, automotive original equipment manufacturers, and electric vehicle supply equipment developers, allowing for sensible design and management of the LIB pack to satisfy target life requirements in a cost-effective way. This presentation will discuss some of the implications associated with different charging protocols, e.g., alternating current level 2 (AC L2), direct current fast charging (DCFC), and combined AC L2 and DCFC, on cells as well as full packs at different temperatures. The effect of delayed fast charging, in which charging completes shortly before the next discharge, on the battery SOH will also be shown. Finally, pack design considerations that require understanding of aspects extending beyond scaling the performance at the cell level will be discussed. References S. M. Knupfer, R. Hensley, P. Hertzke, P. Schaufuss, Electrifying insights: How automakers can drive electrified vehicle sales and profitability, McKinsey & Company (2017)B. Nykvist, M. Nilsson, Rapidly falling costs of battery packs for electric vehicles, Nat. Clim. Change 5, 329-332 (2015)N. Lutsey, S. Searle, S. Chambliss, A. Bandivadekar, Assessment of leading electric vehicle promotion activities in United States cities, Int. Counc. Clean Transp., July 2015.M. McCarthy. California ZEV policy update- SAE 2017 Government/Industry meeting presentation, Washington DC, Jan 2017.S. Ahmed, I. Bloom, A. N. Jansen, T. Tanim, E. Dufek, A. Pesaran, A. Burnham, R. B. Carlson, F. Dias, K. Hardy, M. Keyser, C. Kreuzer, A. Markel, A. Meintz, C. Michelbacher, M. Mohanpurkar, P. A. Nelson, D. C. Robertson, D. Scoffield, M. Shirk, T. Stephens, R. Vijayagopal, J. Zhang, Enabling fast charging – A battery technology gap assessment, J. Power Sources, 367, 250-262 (2017)